Photo-cathode-based accelerator systems and light sources are driven by drive lasers. The drive lasers associated with these systems must meet rigid specifications regarding such features as pulse length, wavelength, power, repetition rate, beam quality, intensity, and phase stability. Such accelerator systems consist of many complicated components requiring high precision and care in tuning the instrument and requiring a very high degree of careful implementation of a well-developed procedure for initial instrument tuning and set-up.
A drive laser plays an important role in the tuning and set up. One of the most basic and important requirements on a drive laser is to provide macro-pulses having various macro-pulse lengths and repetition rates. Accelerator systems are set up at low current (which means low pulse repetition rate at selected pulse length(s)) before operating the accelerator in high current modes. Since drive laser micro-pulses are MHz to GHz quasi-CW (continuous wave) pulse trains generated by pico-second (ps) mode-locked oscillator and amplifiers, fast electro-optic (EO) devices such as Pockels cells are usually used to chop out a macro-pulse with a selected time duration and repetition rate. The macro-pulse length is typically determined solely by the “on” and “off” time of the Pockels cell which is driven by a high voltage electrical pulse. Leakage through the Pockels cell (i.e. which causes unwanted ghost pulses) always exists in the prior art due to the limited contrast of the Pockels cell. Ghost pulses are problematic to the operation of the accelerator as they interfere with the electron beam diagnostics and may cause excessive radiation along the beam line. Accordingly, eliminating ghost pulses or alternatively minimizing ghost pulses to below an acceptable threshold level would be highly desirable.
One method of increasing pulse contrast (i.e. eliminating ghost pulses) is to use a plurality of Pockels cells. However, using multiple Pockels cells has several disadvantages including substantial cost, alignment difficulties and laser power loss.
Alternatively, a Pockels cell may be used in combination with a mechanical shutter as blocking a laser pulse in an effective method of eliminating an unwanted pulse. In a system having a Pockels cell and a fast mechanical shutter, the Pockels cell provides a gated time window with fast leading edge and falling edges shorter than the interval between two macro-pulses. Typically this window is a few nano-seconds. The mechanical shutter provides a wider window that eliminates the ghost pulses outside the Pockels cell window and improves the overall pulse contrast. While the use of a mechanical shutter can significantly improve the overall pulse contrast (i.e. remove a portion of the ghost pulses), the shutters of the prior art address the ghost pulses outside the window but do not eliminate ghost pulses inside the exposure window. The ghost pulses inside the exposure window remain and continue to adversely impact the pulse contrast.
Theoretically, the mechanical shutter's window can be shortened to a level of a few milliseconds, but even if the window is shortened to the limit, ghost pulses remain inside the exposure window. Further the macro-pulse length generated by a Pockels cell is variable and can be as short as sub micro-seconds. For very short macro-pulse lengths, the ghost pulses can become the principle feature of the contrast due to the relatively fewer main signal pulses.
Accordingly, a practical method for creating enhanced pulse contrast ratios for drive lasers and photo-cathode-based electron accelerators is needed.